For diagnostic and other purposes, it is desired to display certain low level electrical signals originating in the body which are of such low potential that they cannot be displayed with conventional instrumentation. A particular field of concern is the study of the cardiac conduction system activity.
An important part of the clinical evaluation of the function of the cardiac conduction system is a method, introduced by Scherlag, for studying the His bundle activity. This method is described by Scherlag in Circulation Vol. 39, page 13, 1969. The Scherlag method was carried out during right heart catheterization and, because of its invasiveness, its application has been limited. Accordingly, a non-invasive technique for detection of the cardiac conduction system activity (from the body surface) is needed.
The His bundle activation voltage has a low level of transmission to the chest wall, typically below 10 microvolts and most commonly about 2 to 5 microvolts. At this low level, it cannot be detected and displayed with conventional electrocardiographic techniques which utilize relatively low gain amplification. Higher amplification in otherwise conventional electrocardiographs result in the signal being obscured by noise originating in the amplifiers, ambient electrical fields, musculoskeletal activity and electrode-skin interface. Such noise components have random characteristics and can be eliminated by signal averaging and filtering. Systems have been developed with improved signal-to-noise ratio by averaging several consecutive cardiac cycles for detection of the electrical activity occurring between the end of the P wave (atrial activity) and the QRS complex (ventricular activity). This signal averaging technique has the shortcoming that it cannot be used for evaluation of individual cardiac cycles. Thus, it is not useful for a study of multifocal arrhythmias or transient events such as intermittent conduction delays or blocks.
A general object of this invention is to provide apparatus for displaying the activity of the cardiac conduction system from a single cardiac cycle on a "beat-to-beat" basis.